In the past procedures have been proposed for converting an acrylic fibrous precursor to an amorphous carbon form or to a graphitic carbon form which retains essentially the same fibrous configuration as the starting material. The acrylic fibrous material is first thermally stabilized, and then carbonized.
The thermal stabilization of an acrylic fibrous material in an oxygen-containing atmosphere is well known in the art and involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure. The cyclization reaction is exothermic in nature and must be controlled if the fibrous configuration of the acrylic material is to be preserved. Accordingly, stabilization procedures commonly proposed are conducted for many hours (e.g. at 220.degree.C. for 3 to 7 hours, or more). During the carbonization reaction elements in the stabilized fibrous material other than carbon, e.g. nitrogen, hydrogen, and oxygen are expelled. The term "carbonized fibrous material" as used herein is defined to be a material consisting of at least about 90 per cent carbon by weight, and preferably at least about 95 per cent carbon by weight. Depending upon the conditions under which the carbonized fibrous product is processed, substantial amounts of graphitic carbon may or may not be present in the same as determined by the characteristic x-ray diffraction pattern of graphite.
The achievement of uniformly superior mechanical properties in carbon fibers, such as tensile strength and initial modulus, has been an elusive goal when employing processes of the prior art. For instance, heretofore, it has been proposed that high longitudinal tensional forces be exerted upon a carbonaceous fibrous material during the formation of graphitic carbon. Unfortunately, such processes which operate under high tensions tend to be unstable and have a tendency to fail because of fiber breakage which may be traced at least in part to incipient flaws or voids present in the fibrous material. For a reliable commercial operation, such process failures cannot be tolerated.
Carbon fibers are being increasingly proposed for utilization as a reinforcing medium when embedded in a suitable matrix to form a strong lightweight structural component. Such composites find particular applicability in aerospace applications. There is accordingly a demand for high strength graphitic fibrous materials having uniform properties which may be reliably looked to for the desired reinforcement.
It is an object of the invention to provide an improved process for the conversion of certain acrylic fibrous materials to high strength graphitic fibrous materials.
It is an object of the invention to provide a process for the production of a graphitic fibrous material possessing essentially uniform mechanical properties, i.e. a high tensile strength and a high initial modulus.
It is another object of the invention to provide a process for the conversion of certain acrylic fibrous materials to graphitic fibrous materials in which the carbonization/graphitization portion of the process may be conducted on a reliable and stable basis wherein the desired tenacity is achieved without the necessity of resorting to the exertion of high longitudinal tensions upon the preoxidized fibrous material undergoing conversion.
It is another object of the invention to provide a process for the production of high tenacity graphitic fibrous materials which is relatively insensitive to variations in longitudinal tension during the carbonization/graphitization portion thereof.
It is a further object of the invention to provide a relatively rapid process for production of graphitic fibrous materials which does not require that the acrylic precursor be water washed to remove residual solvent prior to its utilization.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.